PDF tab order

Silktide checks that pages with links or form fields declare the order in which those elements should be reached. A PDF page can either follow the document's own reading order or fall back on the order the elements were added to the file, and only the first is predictable.

The elements in question are : links, form fields, comments, and anything else a keyboard user moves between. A single setting on each page decides which order applies, and the accessible answer is to follow the structure.

Why this matters

Keyboard users reach interactive elements by pressing the Tab key, and the order they arrive in is the order the document dictates. When a page follows its structure, that order matches the way the page reads, so it is where anyone would expect to go next.

When a page does not, the order is whatever sequence the elements happen to be stored in, which reflects how the file was assembled rather than how it is laid out. In practice, this means tabbing from the top of a form to a link in the footer, back to the middle of page two, then to a field near where you started. On a form it is worse than disorienting: fields get filled in the wrong order, and someone who cannot see the whole page has no way to know they have skipped one. Long forms are where this most often ends in an incomplete submission.

The effect is invisible to anyone using a mouse, which is why it survives review so often. Nothing about the page looks wrong, and nothing goes wrong until someone uses the keyboard.

How to fix it

Export from a tool with accessibility or PDF/UA output enabled, which sets this on every page automatically. This is the reliable fix, because the setting is per page and a long document has many.
Or set it in Acrobat Pro: in the Page Thumbnails panel, select the pages, then use Page Properties > Tab Order and choose "Use Document Structure".
Get the underlying structure right too, since following the structure only helps when the structure reflects the page. A form whose jumps around will tab in that same order.
Or run Silktide remediation, which sets a structure-based tab order as it rebuilds the document. See How PDF remediation works.

How Silktide tests this

Find the PDF documents linked from your website, and analyse each unique file once no matter how many pages link to it.
Find the pages that carry annotations, since a page with none has nothing to order.
Check whether each of those pages declares that tab order should follow the document structure.
Report the document when one does not, once per document no matter how many pages are affected.

Troubleshooting

Tabbing works fine when I try it

It may well do so, by coincidence. When elements were added in roughly the order they appear, the fallback order looks correct, and it stays correct until someone edits the document: adding one field, replacing a page, or merging in a section reorders everything after it with no visible sign. Declaring the order is what makes it survive editing.

The page only has one link

Then nothing is out of order today. The setting is still worth having, for the same reason: it costs nothing now and stays right when the page gains a second link.

My form fields are in the wrong order even with this set

Then the document's structure has them in that order, and the structure is what needs correcting. Tab order following the structure is only as good as the structure it follows. See Text and reading order.